Powered snowthrowers of the walk behind type are well known. Such snowthrowers usually have a housing supported by wheels which carries a snowthrowing impeller or paddle at the front thereof for engaging and throwing snow. In addition, a handle assembly extends upwardly and to the rear from the housing so that the operator can hold the handle assembly and walk behind the snowthrower as it moves over the ground. In some cases, for example in a two stage snowthrower, the wheels which support the housing for movement over the ground can be selectively coupled to the power source to comprise traction or drive wheels. The wheels on a single stage snowthrower are not usually powered in this sense. Any self-propelling action usually comes from the engagement of the snowthrowing paddle with the ground.
Most two stage snowthrowers that utilize traction wheels also use a transmission for driving the traction wheels at different speeds and in different directions, i.e. either forward or reverse. In the two stage snowthrowers manufactured by The Toro Company, Minneapolis, Minn., the assignee of the present invention, two separate controls are provided to engage the traction wheels. The first control is a shift handle which the operator uses to select the forward speed and the direction of movement for the traction wheels. The second control is a deadman's handle which the operator must squeeze and keep closed to apply power to the traction wheels. In addition, in most snowthrowers of this type, the traction wheels are located approximately beneath the center of gravity or slightly behind the center of gravity of the snowthrower.
Two stage snowthrowers of the type described above are very effective in removing and throwing snow. However, the placement of the traction wheels in a fixed position approximately beneath the center of gravity of the snowthrower can, in certain limited circumstances, lead to some difficulties in using the snowthrower. This occurs primarily in wet, heavy or hard packed snow where the front of the snowthrower is attempting to push into the snow pack. In such conditions, the front of the snowthrower tends to climb up over the snow rather than bite into the snow. The operator then has to lift upwardly on the handle assembly and apply a fairly continuous upward pressure in many cases to keep the front of the snowthrower down. This can be tiring and difficult to do. Moreover, lifting upwardly takes weight off the wheels thereby losing traction. However, in most other snow conditions, i.e. smaller amounts of snow or loose packed snow, the traction wheel placement beneath the center of gravity is generally ideal. In other words, the traction wheels have enough tractive force to operate properly and bite into the snow, and also allow the snowthrower to be easily maneuvered in turning it from side to side.
Some prior art snowthrowers utilize a traction system which is different from that just described. Rather than use a pair of traction wheels, one on either side of the snowthrower, these models use instead elongated tracks with the tracks being supported on front and rear pulleys or sprockets. The front pulley for the sprocket is located generally beneath the center of gravity of the snowthrower, in a location similar to that of the conventional traction wheel, while the rear pulley is located considerably to the rear of the front pulley. The track is made of any suitably pliable material and may include ribs or cleats for engaging the snow in a manner similar to that of a snowmobile track. In addition, the rear pulley on which the track is supported can be vertically adjusted relative to the snowthrower housing to change the angle or inclination which the track makes with the snowthrower housing, and hence also with the ground.